Reaction Mechanisms

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Reaction Mechanisms

• Chapter 12, Section 6

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Reaction Mechanisms

• The sequence of events that describes the actual
  process by which reactants become products is
  called the reaction mechanism.

NO2 CO ? NO CO2 Rate kNO22
Proposed Mechanism Step 1 NO2 NO2 ??? NO3
NO Step 2 NO3 CO ??? NO2 CO2
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Reaction Mechanisms

• Reactions may occur all at once or through
  several discrete steps.
• Each of these processes is known as an elementary
  reaction or elementary process.

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Reaction Mechanisms

• The molecularity of a process tells how many
  molecules are involved in the process.
• Termolecular elementary reactions seldom occur.

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Multistep Mechanisms

• In a multistep process, one of the steps will be
  slower than all others.
• The overall reaction cannot occur faster than
  this slowest, rate-determining step.

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Reaction Mechanisms must satisfy 2 requirements

1. The sum of the elementary steps must give the
   overall balanced equation.
2. The mechanism must agree with the experimentally
   determined rate law.

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Slow Initial Step
NO2 (g) CO (g) ??? NO (g) CO2 (g)

• The rate law for this reaction is found
  experimentally to be
• Rate k NO22
• CO is necessary for this reaction to occur, but
  the rate of the reaction does not depend on its
  concentration.
• This suggests the reaction occurs in two steps.

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Slow Initial Step

• A proposed mechanism for this reaction is
• Step 1 NO2 NO2 ??? NO3 NO (slow)
• Step 2 NO3 CO ??? NO2 CO2 (fast)
• Does it make sense?
• What is the intermediate in this reaction?

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Fast Initial Step
2 NO (g) Br2 (g) ??? 2 NOBr (g)

• The rate law for this reaction is found to be
• Rate k NO2 Br2
• Because termolecular processes are rare, this
  rate law suggests a two-step mechanism.

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Fast Initial Step

• A proposed mechanism is

Step 2 NOBr2 NO ??? 2 NOBr (slow)
Step 1 includes the forward and reverse reactions.
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Fast Initial Step

• The rate of the overall reaction depends upon the
  rate of the slow step.
• The rate law for that step would be
• Rate k2 NOBr2 NO
• But how can we find NOBr2?

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Fast Initial Step

• NOBr2 can react two ways
• With NO to form NOBr
• By decomposition to reform NO and Br2
• The reactants and products of the first step are
  in equilibrium with each other.
• Therefore,
• Ratef Rater

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Fast Initial Step

• Because Ratef Rater ,
• k1 NO Br2 k-1 NOBr2
• Solving for NOBr2 gives us

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Fast Initial Step

• Substituting this expression for NOBr2 in the
  rate law for the rate-determining step gives

k NO2 Br2
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Catalysts

• A catalyst speeds up a reaction without being
  consumed.
• Identify the catalyst in the reaction mechanism
  below
• Cl(g) O3(g) ? ClO(g) O2(g)
• O(g) ClO(g) ? Cl(g) O2(g)
• How is it different than an intermediate?